My experiences are purely with Indo-pacific tarpon.
1) The water can be completely fresh, although a pH on the alkaline side does help. A little salt can also hold back fungal infections, which can happen - these fish are pretty skittish at first, and they're fast. Lunging around a tank can knock off scales pretty easily.
2) They will eat almost anything. Pellets are cheap and easy, although they'll eat small fish, prawns etc. too. Bear in mind they're carnivores, so a diet of algae wafers for example wouldn't be ideal.
3) Standard tropical temperatures are fine - try not to go too far below 24 and not far above 30. These fish will tolerate very warm water as they can breathe air and will still do so quite regularly, even in a heavily aerated tank.
4) Steer away from aggressive, extremely predatory tankmates. These fish are fairly flighty and not confrontational at all. I wouldn't reccommend having them with large cichlids for example. They're certainly not territorial, but will eat anything small enough to fit in their mouth. Particularly if it wasn't there before they were.
5) I don't think they mind if they're alone or in groups, although they do school quite well, and do seem to enjoy each other's company.
6) They've been quite hardy in my experience. Others have had different experiences, but in hard, alkaline water with an over-gunned filter, I've never had any issues.
7) Can't help you with cost... i caught my own (Living in Tropical North Queensland in Australia has its benefits).
8) They'll get as big as you can get them. This all depends on tank/pond size, filtration, feeding etc. They do have a growth potential of 1.5m, but they do appear to commonly reach 50cm in captivity. Growth rate's appreciable as specimens you'll likely get will be juveniles. They eat constantly, and provided with enough food, they'll get large very quickly.
Please also bear in mind - These fish have a tendency to jump! Keep the lid down firmly.
Otherwise, they're quite neat fish. Mine hand-fed within one week of being in my tank after having been caught on fly gear. Honestly didn't expect that to happen so quickly.
Once they get to know you and the tank, you'll find they're nowhere near as flighty too. Chalceus (If you've ever kept them) are MUCH flightier.
